In an image forming device such as an electrophotographic device and an electrostatic recording device, first, an electrostatic latent image famed on the photoconductor is developed with a toner. Next, as needed, a toner image thus formed is transferred onto a transfer material such as a paper sheet and then fixed thereon by various methods such as heating, pressurization or solvent fume.
In the field of such an image forming device, a digital full-color copying machine and a digital full-color printer have been put to practical use. A digital full-color copying machine produces a full-color image as follows. First, an original color image is subjected to color separation with blue, green and red filters; an electrostatic latent image corresponding to the original color image, which is composed of dots that are 20 to 70 μm in diameter, is developed with yellow, magenta, cyan and black toners; and a full-color image is famed using the subtractive color mixing effect.
Recently, there is an increasing demand for full-color images with high image quality and high resolution. Especially, to increase color reproducibility, it is hoped that an image can be printed in the same hue as ink printing.
As a color pigment for a yellow toner, for example, a disazo pigment as typified by C.I. Pigment Yellow 12, 13 and 17, and a monoazo pigment as typified by C.I. Pigment Yellow 74, 97 and 98 have been generally used.
Patent Literature 1 discloses a yellow toner for developing electrostatic images, the toner comprising a mixed pigment of C.I. Pigment Yellow 74 and C.I. Pigment Yellow 185 as a colorant. In Patent Literature 1, it is described that by mixing C.I. Pigment Yellow 185 with C.I. Pigment Yellow 74, which is a reddish-yellow pigment, the resulting toner has not only the same hue as offset inks, but also low structural viscosity in an organic solvent; therefore, the toner has no difficulty in granulation in water and has better secondary color reproducibility.
Patent Literature 2 discloses a yellow toner comprising C.I. Pigment Yellow 74 and C.I. Solvent Yellow 162. In Patent Literature 2, it is described that by containing these colorants, the toner obtains satisfactory color tone that is obtained in the case of incorporating C.I. Pigment Yellow 74 solely in a toner, and high coloring power of C.I. Solvent Yellow 162.
Patent Literature 3 discloses a yellow toner comprising C.I. Pigment Yellow 74 and C.I. Pigment Yellow 155. In Patent Literature 3, it is described as follows: by allowing C.I. Pigment Yellow 74 and C.I. Pigment Yellow 155 to coexist in a toner material, the pigments inhibit each other's crystal growth even if they are heated during kneading and mixing, and the dispersion of the pigment particles are highly maintained; therefore, a yellow toner that has high coloring power, sharp color tone, satisfactory color mixing property and excellent transparency, is obtained.
Patent Literature 4 discloses a yellow toner comprising C.I. Pigment Yellow 214 and C.I. Pigment Yellow 139 as colorants. Patent Literature 4 describes that a toner excellent in fixability and color properties is obtained by interaction between the amino groups of C.I. Pigment Yellow 214 and the carbonyl groups of C.I. Pigment Yellow 139.